For a variety of reasons, plant species may be intentionally bred. For example, in some applications plant species are intentionally bred to form hybrid plant species. In some applications, hybrid plants are bred to exhibit various desirable traits. Such traits may include, for example, resistance to heat and drought, resistance to disease and insect damage, improved yield characteristics, and improved agronomic quality. In general, plants may be capable of self-pollination, cross-pollination, or both. Self-pollination describes pollination using pollen from one flower that is transferred to the same or another flower of the same plant. Cross-pollination describes pollination using pollen delivered from a flower of a different plant from a different family or line.
Plants that have been self-pollinated and selected for many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny. A cross between two different homozygous lines produces a uniform population of hybrid plants that may be heterozygous for many gene loci. A cross of two plants each heterozygous at a number of gene loci will produce a population of heterogeneous plants that differ genetically and will not be uniform.
Maize (Zea mays L.), often referred to as corn in the United States, can be bred by both self-pollination and cross-pollination techniques. Maize has separate male and female flowers on the same plant. The male flowers are located on the tassel and the female flowers are located on the ear. Natural pollination occurs in maize when wind blows grains of pollen from the tassels to the silks that protrude from the tops of the ears.
The development of a hybrid maize variety in a maize seed production program may involve three steps: (1) the selection of plants from various germplasm pools for initial breeding crosses; (2) self-pollination of the selected plants from the breeding crosses for several generations to produce a series of inbred lines, which, individually breed true and are highly uniform; and (3) crossing a selected inbred line with an unrelated inbred line to produce the hybrid progeny. After a sufficient amount of inbreeding successive filial generations will merely serve to increase seed of the developed inbred. Preferably, an inbred line should comprise homozygous alleles at about 95% or more of its loci.
During the maize inbreeding process, vigor of the line may decrease. Vigor may be restored when two different inbred lines are crossed to produce the hybrid progeny. An important consequence of the homozygosity and homogeneity of the inbred lines is that the hybrid between a defined pair of inbreds may be reproduced indefinitely as long as the homogeneity of the inbred parents is maintained. Once the inbreds that create a superior hybrid have been identified, a continual supply of the hybrid seed can be produced using these inbred parents and the hybrid corn plants can then be generated from this hybrid seed supply.
Accordingly, development and production of maize seed may require controlled pollination at one or more steps, as described above.